Many types of devices include a body which rotates about an axis. For example, devices such as motors, pumps or the like, include a shaft which rotates.
One specific device is a canned motor pump. The canned motor pump is commonly known and includes a stator and rotor each enclosed in a respective can. As a result of being enclosed, however, the rotor shaft cannot be viewed easily. This makes it difficult to detect bearing wear, shaft movement, shaft rotation, etc. In addition, canned motor pumps typically are designed to be immersed in toxic and/or corrosive liquid for pumping such liquid. Canned motor pumps use a sleeve type bearing to support a coercion resistant shaft, where the process fluid is a lubricant. In this environment, the sleeve bearings wear at rates governed by the lubricity of the fluid being pumped and the amount of particle contaminate contained in the fluid. Therefore, it is of interest to pump users to monitor the condition of the bearings on a continuing basis.
There are many possible methods which potentially could be used on a commercial basis for measuring the physical position of the rotating shaft in a canned motor pump. Those methods can be generally classified as: optical, sonic, capacitance, and eddy current (high frequency inductance) based methods. Nevertheless, each of these methods suffer from inherent limitations. For example, optical and eddy current measurement probes require breaching the pressure containment can with the probe. Moreover, these types of probes would also have to be able to withstand the rigors a very hostile chemical environment, i.e., the process fluid. A fundamental limitation of the capacitance technique is that the process fluid becomes the dielectric of the capacitor, and therefore the characteristics of the process fluid would govern the quality of the measurement. The imprecision of sonic probes when measuring distances in mils (0.001 inch), plus the necessary immersion in the process fluid also eliminates these devices as viable alternatives.
Therefore, the operational criteria for a sensor for measuring the position of the shaft in a canned motor pump is:
1. Capable of resolving shaft position within mils. PA1 2. Unaffected by the process fluid environment. PA1 3. No penetration of the either the stator or rotor cans.
There have been attempts in the past to provide a sensor which meets the above criteria, but such attempts have met with limited success. For example, U.S. Pat. No. 4,924,180 discloses a device for detecting the position of a rotating shaft in a sealed pump. However, the device only detects radial movement of the shaft. The device does not detect axial movement of the shaft. Furthermore, the device does not detect the direction of radial movement or the direction of rotation, each of which can be equally important.
In view of the aforementioned shortcomings associated with existing devices for detecting the position of a rotating shaft, there is a strong need in the art for a system which detects movement of a rotating shaft in both axial and radial directions with high precision. In addition, there is a strong need in the art for a system which detects the direction of the axial and radial movement of the shaft as well as the direction of rotation. Furthermore, there is a strong need in the art for a system which is flexible enough to be used in different environments such as a canned motor pump.